A variety of normally gaseous or liquid blowing agents have been proposed for olefinic or styrenic polymers, including virtually all of the common atmospheric gases and lower hydrocarbons.
Alkenyl aromatic foams, particularly polystyrene foams in sheet form, are presently being made from a number of blowing agents which have many undesirable characteristics. Volatility, flammability, poor thermoforming qualities, brittle physical properties, high cost, or an adverse affect to the ozone layer are just a few. Examples of the blowing agents that produce these characteristics in the production of polystyrene foam would include the aliphatic hydrocarbons, and fully (or partially) halogenated hydrocarbons.
For polystyrene, for example, the C.sub.4 -C.sub.6 alkanes have gathered widespread acceptance, especially pentane. Following a typical extrusion foaming step, the stock material is ordinarily aged before thermoforming into containers or the like. During aging, the foam cells and polymeric matrix become partially depleted of volatile hydrocarbons, which enter the atmosphere. However, potential atmospheric contamination by these by-products of foam manufacture has led workers to seek non-polluting alternative blowing agents, such as the usual atmospheric gases, e.g., nitrogen and carbon dioxide, and combinations of atmospheric gases with organic gases, e.g., the lower hydrocarbons or the freons.
In the prior art, both atmospheric gases, per se, and combinations of atmospheric and organic gases have been disclosed as blowing agents for alkenyl aromatic polymers.
Australian Patent Application No. 52724/79, published Canadian Patent Application No. 2,022,501 and published European Patent Application No. 0,411,923 all disclose blowing agents consisting of carbon dioxide for alkenyl aromatic or styrenic polymers. The resulting foamed products are said to be flexible and/or have improved tensile elongation properties. However, the production rates of the processes are generally low, on the order of less than 200 lbs./hr., and also have generally low post-expansion properties, on the order of 50% or less. In addition, these processes require relatively high extrusion temperatures, on the order of 130.degree. C. to 155.degree. C. Thus, these processes are not very economical.
In co-pending patent application Ser. No. 07/891,866, there are disclosed processes for producing polystyrene foams utilizing 100% of atmospheric gas, e.g. carbon dioxide and/or nitrogen, which can be effected at a much lower extrusion temperature, i.e. on the order of about 120.degree. C., utilizing in the melted polymer an additive comprised of a masterbatch mix containing alpha-methyl polystyrene, a rubbery block copolymer, a solid blowing agent comprised of an encapsulated combination of monosodium citrate and sodium bicarbonate, white mineral oil, and silica.
U.S. Pat. Nos. 4,344,710 and 4,424,287 disclose blowing agents which are combinations of liquid carbon dioxide and liquid aliphatic, or fully (or partially) halogenated hydrocarbons. These patents state that the use of atmospheric gases, including 100% carbon dioxide or nitrogen as blowing agents has not been successfully employed, giving as a reason the extreme volatility of these gases, and further state that the use of these materials is said to produce corrugation and surface defects in the sheet product. These two patents disclose that a combination of atmospheric and organic gases, in an alkane: CO.sub.2 feed ratio in the range of 3:1 to 1:1 by weight, can be used, with the total amount of blowing agent combination being in the range of 2.5 to 10 parts per 100 parts by weight of thermoplastic resin. As nucleating agents for the foamed products, the patents disclose the use of a mixture of sodium bicarbonate and citric acid. The process temperatures needed for extrusion of the foam are again quite high, on the order of 150.degree. C.
U.S. Pat. No. 4,424,287 further discloses that the foams prepared with the combination of blowing agents exhibit the advantage of reduced atmospheric emissions upon aging without, however, any data to this effect, merely stating that the reduction in pollutant (i.e. the hydrocarbon blowing agents) is greater than the expected reduction due to the corresponding decrease in organic blowing agent use. The only rationale provided in U.S. Pat. No. 4,424,287 for the reduced hydrocarbon emissions is the ability of the foamed sheet product to be immediately thermoformed, thereby reducing the need for aging of the foamed sheet product.
U.S. Pat. No. 4,419,309 discloses the use of two foaming agents; the first being introduced into a molten thermoplastic resin under higher pressure, with the first foaming agent being selected from a low molecular weight aliphatic hydrocarbon, a low molecular weight halocarbon and mixtures thereof, and the second foaming agent being introduced under lower pressure, with the second foaming agent being selected from carbon dioxide, water vapor and mixtures thereof, to cause foaming of the melted thermoplastic resin. Again, the extrusion rates are low, on the order of 150 lbs./hr., and the extrusion temperatures are high, on the order of 290.degree. F.-320.degree. F.
U.S. Pat. Nos. 4,916,166 and 5,011,866 disclose alkenyl aromatic thermoplastic synthetic resinous elongated foam bodies having a machine direction, a transverse direction and closed, non-interconnecting gas-containing cells, which are prepared using, preferably at least 70% by weight of 1,1-difluoro-1-chloroethane (U.S. Pat. No. 4,916,166) and requiring the use of at least 70% by weight of 1,1,1,2-tetrafluoroethane or 1,1,1-trifluoroethane, based on the total weight of blowing agent mixture weight (U.S. Pat. No. 5,011,866), and using as a second blowing agent up to 30 weight percent (of the blowing agent in an amount of mixture) chemical or physical blowing agents, including water, 1-4 carbon aliphatic hydrocarbons, carbon dioxide, or other hydrogen-containing chlorofluorocarbons (HCFCs) such as chlorodifluoromethane (HCFC-22).
U.S. Pat. No. 4,916,166 discloses that the amount of carbon dioxide is limited to no more than about 6% by weight and that extruded articles having densities between 2.4 and 5.0 pounds per cubic foot may be obtained only by extrusion at a die temperature of about 118.degree. C. or less. The extrusion rate at this temperature should necessarily be quite low, although the patent is silent on this point. The specific examples of U.S. Pat. No. 4,916,166 show that extruded foam articles having densities of less than 2.4 pounds per cubic foot are obtained only upon extrusion above 118.degree. C., and these are obtained utilizing blowing agents which contain only about 2.7% by weight carbon dioxide based upon 100% by weight of the blowing agent mixture.
U.S. Pat. No. 5,011,866 discloses alkenyl aromatic thermoplastic synthetic resinous elongated foamed products having densities of from about 1 to about 6 pounds per cubic foot which have a plurality of closed non-interconnecting gas-containing cells, with the limitation that the cells contain at least 70% by weight of either 1,1,1-trifluoroethane or 1,1,1,2-tetrafluoroethane. U.S. Pat. No. 5,011,866 likewise prefers the use of less than 6% carbon dioxide as a component in a blowing agent mixture although some examples show the use of about 9% carbon dioxide.
Thus, there still exists a need in the art for procedures for the production of alkenyl aromatic foams which utilize combinations of atmospheric and organic gases as blowing agents and having an increased amount of atmospheric gas, such as carbon dioxide or nitrogen. There also still exists a need in the art for such alkenyl aromatic foams which can be produced at increased temperatures and increased extrusion rates for a given density. Still further, there exists a need in the art for alkenyl aromatic foams having an increased percentage of closed, non-interconnected cell structure, increased post-expansion properties, and increased retainment of blowing agent within the cell structure of the alkenyl aromatic foam.
These and other needs still remaining in the alkenyl aromatic foam art are met and satisfied by applicants' present invention, described below.